She Researched What's Hiding In Conventional Fabrics. Then She Replaced His Underwear.

Polyester Traps Heat Where It's Least Wanted. The Numbers Back It Up.

Your testes sit outside your body for one reason: they need to run cooler than your core — roughly 2–6°C cooler — to do their job. Research summarized in Reproductive BioMedicine Online lays it out plainly: when scrotal temperature climbs, sperm production and quality drop. One Danish study (Hjollund et al., 2000) linked roughly a 1°C rise in scrotal temperature to about a 14% reduction in sperm concentration.

Polyester is a plastic. It doesn't breathe the way cotton does, so it traps the heat you generate training, working, or just sitting at a desk. That's the part that needs no exaggeration. Organic cotton breathes, wicks, and lets things cool back down.

656 Men. One Question: What Do You Wear? The Loose, Breathable Crowd Won.

In 2018, researchers at the Harvard T.H. Chan School of Public Health and Massachusetts General Hospital published a study in Human Reproduction looking at 656 men and the underwear they usually wore.

The study was about fit and airflow, not fabric specifically — and it was observational, in a fertility-clinic population. We're not going to overstate it. But the direction is clear and it lines up with the heat data: give things room and air, and the numbers move the right way. It's also why Gotchies comes in three cuts. Boxer briefs are the supportive pick Aspen chose. Boxers are the loosest option, and the one with no elastane at all.

Plastic Is Showing Up Where It Shouldn't. Your Skin Isn't The Barrier You Think It Is.

Polyester and a plastic water bottle are both made from petroleum-derived plastics, and synthetic textiles shed microplastic fibers every time they're worn and washed. That's the kind of thing that can hide in a conventional fabric. Two studies from 2024 made the connection harder to wave away:

Here's the honest version: finding microplastics in testes is not the same as proving they cause infertility, and the exact dose you absorb from a pair of underwear hasn't been measured. What has been measured is that these compounds are endocrine-active in the lab, that they get through skin, and that they're in the fabric. You can wait for the twenty-year study — or you can just not wear the plastic. Aspen chose option two.

In 1992, Polyester Underwear Was Tested As A Male Contraceptive. It Worked On Every Man.

Dr. Ahmed Shafik had 14 healthy men wear a polyester scrotal sling continuously for a year. Every one of them stopped producing sperm — on average by day 140 — and every one recovered about five months after removing it. The paper, “Contraceptive efficacy of polyester-induced azoospermia in normal men,” was published in the journal Contraception and has been sitting on PubMed ever since.

A sling worn 24/7 is a more extreme setup than a pair of boxer briefs, and it was 14 men — we'll say that out loud so you don't have to wonder. But it's the reason a whole generation of researchers started asking what synthetic fabric does against that skin. Thirty years later, nobody in Big Underwear has brought it up.

You don't need certainty to make the comfortable, verifiable choice. GOTS-certified organic cotton removes the question entirely.